The invention relates to a toroidal choking coil comprising a fixing device to enable the toroidal choking coil to be attached to electric appliances or the like. The fixing device comprises a hoop-like collar, fasteners resting on the collar for attaching the choking coil onto fastening surfaces of the appliances or the like.
The invention also relates to a method for manufacturing such a toroidal choking coil.
A toroidal choking coil is an electric, technical component wound around an annular core that usually comprises one or more adjacent iron rings; more precisely, it is a coil used for filtering interference that occurs in electric current. A fixing device of the toroidal choking coil refers to a device that enables the toroidal choking coil to be attached to an electric appliance or the like.
According to a known solution, a pre-wound toroidal choking coil is cast in cast resin, the mold used in the casting being provided with fasteners that enable the toroidal choking coil to be attached to an appliance.
However, this solution is problematic in various ways. Firstly, several different molds are needed to carry out castings of different sizes and shapes, which is expensive and impractical. Secondly, such a cast piece takes a lot of space and is relatively heavy. The cast resin forms a closed structure around the choking coil, decreasing heat evaporation, which results in reduced cooling of the choking coil. The specific heat coefficients of cast resin and the iron rings used in the core of the choking coil as well as the electric conductors differ from each other, which means that different thermal expansion of the different parts of such a closed component incurs a risk that the component might be damaged.
Another known solution is to suspend the toroidal choking coil on the fastening surfaces of the appliance using different types of straplike suspenders. The problem is then the poor reliability of the attachment and the risk that the choking coil might easily start to move, whereby the choking coil is more susceptible to damage; the risk that the moving choking coil might damage other components increases as well.
An object of the invention is thus to provide a toroidal choking coil so as to enable the aforementioned problems to be alleviated.
This is achieved by a toroidal choking coil of the invention characterized by what is disclosed in the claims. To be more precise, the toroidal choking coil of the invention is mainly characterized by what is disclosed in the characterizing part of claim 1.
The invention further relates to a method for manufacturing a toroidal choking coil. The manufacturing method of the invention for manufacturing a toroidal choking coil is mainly characterized by what is disclosed in the characterizing part of claim 11.
The idea underlying the invention is that a fixing device, i.e. a fixing collar, is arranged on the outer surface of the annular core of the toroidal choking coil, i.e. between the core and the winding, the fixing device comprising fastening elements to enable the choking coil to be attached to the appliance or the like. The collar and the fastening elements arranged therein are preferably manufactured from a strip- or sheetlike material, such as a metal band or sheet. The fixing device then requires very little space, not increasing the size of the coil, and the fixing device also allows the coil to be entirely wound. In addition, the locations of the fasteners enabling the collar to be attached to the fastening surfaces of the appliance can be chosen freely, preferably with respect to the location of the coil. A preferred shaping enables the entire fixing device to be manufactured from one piece by bending it appropriately.
Furthermore, the collar is preferably narrower than the core, which means that the outer edges of the collar remain substantially inside the outer edges of the core. The outer edges of the collar are then unable to damage the winding because of dimensional changes in the materials caused by thermal expansion.
The inner surface fastening against the outer rim of the iron rings that form the core of the fixing collar of the invention comprises several adjacent wavefronts transversely located with respect to the outer rim of the ring, consisting of successive valleys and protrusions. In addition, the protrusions on the contact surface that fastens against the iron ring of the fastening collar may comprise flexible tongues extending against the outer surface of the iron ring. The wavelike structure and resilient tongues or a combination thereof of the inner surface of the fixing collar enable the fixing collar to yield and stretch in the longitudinal direction, the changes in the dimensions caused by thermal expansion to be compensated for and an adequately strong attachment to be retained. Secondly, the flexibility of the fixing device and the resulting possibility of expanding the rim of the fixing device may also be utilized in the assembly while installing the fixing device in place around the iron rings, particularly when the ends of the fixing device are attached fixedly to each other e.g. by welding.
Furthermore, the flexible structure of the fixing device enables greater tolerances in the size of the iron rings that form the core. The flexible tongues extending against the outer rim of the ring in the protrusions of the inner surface fastening against the ring of the fixing collar improve the contact of the fixing collar with the surface of the ring if the diameter of the rings varies within the scope of the manufacturing tolerance. The tongues operate such that they yield inwards towards the center of the diameter of the collar when the diameter of the ring is shorter than a collar of medium length, while on the other hand, they yield outwards away from the center of the diameter of the collar when the diameter of the ring is longer than a collar of medium length, thus at most equalling the contact surface of the collar.
The ends of the fixing collar of the toroidal choking coil are immovably locked in place when the collar is tightened in place around the rings that form the core. This is carried out by attaching the ends to each other by preferably clawlike locking elements in the locking device. A conventional nut and bolt attachment may also be used. The ends of the fixing collar may be attached to each other also by welding.
At the locking device, the ends of the collar are arranged to partly overlap each other. After being tightened in place around the core, the fixing device of the toroidal choking coil thus remains tightly in place, and the flexible structure of the fixing collar enables the collar to yield owing to thermal expansion of the core.
The structure of the toroidal choking coil of the invention comprising a collar-like fixing member is light in comparison to a device cast in cast resin, the cooling of the toroidal choking coil operating efficiently as well. It is also cheaper.